The development of an HIV-1 vaccine is dependent on the design of an antigen that will elicit a broad and potent neutralizing response. Only four antibodies have been identified with these desired neutralization properties. and one focus of our proposal is to use X-ray crystallography to determine structures of these antibodies in complex with peptide-based, carbohydrate or small molecule compounds designed for use as potential immunogens. In addition, we will investigate anti-HIV-1 antibodies with type-specific neutralization in order to obtain a better understanding of why anti-HIV antibodies display broad versus type-specific activity. Understanding the structural basis for these differences is critical for the design of an effective immunogen. We will supplement the very limited structural information available for the HIV-1 envelope glycoprotein gp120 by the production and structural study of different monomeric gp120 and trimeric gp140 constructs, alone or in complex with one or more anti-gp120 or anti-gp41 Fabs. The use of Fabs as tools for crystallization will also be exploited for the crystallization and structure determination of the HIV-1 viral infectivity factor (Vif) as a complex with a scFv or Vh domain. We will also determine structures for the gp41 fusion active core as a complex with the conformation specific antibody NC-1 and for the gp41 N-helical region with small molecule fusion inhibitors. These studies all have the common goal of providing essential structural information on HIV-1 proteins and anti-HIV-1 antibodies to help guide efforts in vaccine and inhibitor design. [unreadable] [unreadable]